Reminder letter in the first place should indicate that the recipient had forgotten to make an important act. Most often the reminder letter is used as a reminder of the missed payment
When writing a reminder letter you must tread a careful line. You cannot be sure your client is deliberately frustrating you, he may really have forgotten. So you have to maintain a delicate balance between positively stating your request and still retaining the recipient's goodwill.
This is especially true for the first reminder letter concerning an unpaid invoice. So begin your reminder letter in simple form: just let client know about the forgotten event. Include all the relevant details.
To maintain the client's goodwill immediately presume that this was an honest mistake and make sure the client understands that this letter is only a gentle reminder Confirm your belief that this is the case and that you look forward to receiving the payment in the next few day.
If this is not the first reminder letter, try to prove to the client that you are serious. Use strict legal language. It's important that the words "Notice Before Legal Action" are as large as possible, it won't be ignored and you will be paid.
Below is a sample reminder letter:
Our records indicate that payment on your account is overdue in the amount of [amount].
If the amount has already been paid, please disregard this notice. If you have not yet mailed your payment, please make out your check and place it in the enclosed envelope while this reminder is has your full attention.
Thank you in advance for your anticipated cooperation in this matter.